Free and open-source software is software that grants you certain rights, such as being able to distribute and modify the software, under certain conditions. Much Free and open-source software is release under the GNU General Public License, which allows free modification and distribution, under the condition that any derivatives of the program are under the same license, with the source code also released, thus granting further users the same freedoms as were originally present to modify and redistribute the software. Other Free and open-source software is released under more liberal licenses, where you are free to modify, but are also free to keep your modifications private, i.e. proprietary, if you wish.